Microporous polyolefin multi layer film and preparing method thereof

ABSTRACT

The present invention relates to a multi-layered microporous polyolefin film for a battery separator and a method for preparing the same. The microporous multi-layered film of the present invention has a characteristics to have both the low shutdown temperature conferred by the polyethylene and the high melt fracture temperature conferred by the polypropylene and heat-resistant filler. In addition, it has the high strength and stability conferred by the micropores prepared under wet process and the high permeability and high strength conferred by the macropores prepared under dry process. Therefore, this multi-layered film can be used effectively to manufacture a secondary battery with high capacity and high power.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 the benefit ofKorean Patent Application No. 10-2007-0138028, filed on Dec. 26, 2007,and Korean Patent Application No. 10-2008-0000711, filed on Jan. 3,2008, the entire contents of which are incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a microporous polyolefin multilayerfilm stable in quality and outstanding in thermal stability. Moreparticularly, the present invention relates to a microporous film thathas a feature to have both a low shutdown temperature caused bypolyethylene and a high melt fracture temperature caused bypolypropylene and a heat-resistant filler, as well as both uniformmicropores and high strength/stability characterized in the separatorprepared under wet process and high permeability and high strengthcharacterized in the macropores prepared under dry process. Therefore,this multi-layered film can be used effectively to manufacture asecondary battery with a high capacity and high power.

2. Description of the Related Art

The microporous polyolefin film is widely used to manufacture batteryseparators, separating filters, membranes for microfiltration, and thelike, because it is chemically stable and outstanding in physicalproperties.

The method for preparing a microporous film by using polyolefin is oftenaccomplished under wet process. This procedure is comprised of thesteps: mixing polyolefin and a diluent at high temperature to make asingle phase; cooling to separate the polyolefin phase and the diluentphase; and then, extracting the diluent to generate pores in thepolyolefin. The procedure can be applied to manufacture a thin film sothat it is popularly used for high-capacity, high-power lithium ionsecondary battery, and the like, due to the high strength andpermeability, uniform pores and the evenness of quality.

General method for preparing the porous film by using wet process hasbeen disclosed in U.S. Pat. No. 4,247,498. This method described thesteps: selecting a diluent proper polyethylene; blending the mixture athigh temperature to make a thermodynamically single-phase solution; andcooling to separate the polyolefin phase and the diluent phase so as tofabricate a porous film of the polyolefin.

The lithium ion secondary battery is an outstanding battery very high inthe energy density, but is associated with the risk of explosion causedby short circuit. Therefore, a high level of quality is required for theseparator with the eveness of quality. Further, as the application ofthe lithium ion secondary batteries is extended to hybrid cars and otherfields, a stricter thermal stability requirement is required for theseparator. When the thermal stability decreases, the battery may explodedangerously due to rupture separator when being overheated. It isbecause poor thermal stability of the separator may lead to overheatingof the battery and explosion caused by melting and rupturing separators.

Thermal stability of separator in a battery is determined by shutdowntemperature and melt down temperature.

Shutdown temperature is the temperature at which the micropores of theseparator are closed to shut the electric current when the inside of thebattery is abnormally overheated. Melt down temperature is thetemperature at which the separator is subjected to melt and the electriccurrent flows again when the battery temperature increases beyond theshutdown temperature. To ensure thermal stability of a battery, it ispreferred that the shutdown temperature is low and the melt downtemperature is high. Especially, the melt down temperature is closelyrelated with the battery stability, because it shuts electric currentcontinuously even under an explosive situation.

In order to improve the thermal stability of the separator, three kindsof approaches have been attempted. One is to add an inorganic materialor heat-resistant resin to the polyethylene in order to increase thethermal stability of the separator. Another is to coat a heat resistantmaterial on the surface. The other is to manufacture a multi-layeredseparator containing a heat resistant layer.

U.S. Pat. No. 6,949,315 discloses a method blending UHMW (ultra highmolecular weight) polyethylene with 5-15 weight % of an inorganicmaterial such as titanium oxide to improve thermal stability of theseparator. However, this method is disadvantageous in that, although theaddition of the inorganic material provides the effect of improvingthermal stability, it may lead to poor mixing and nonuniform quality andformation of pinholes resulting therefrom, or poor film propertiesbecause of lack of compatibility at the interface between the inorganicmaterial and the polymer resin. These disadvantages are inevitable in aseparator using an inorganic material.

U.S. Pat. No. 5,641,565 discloses a method of blending a resin havingsuperior heat resistance instead of using an inorganic material. In thismethod, the separator is manufactured by the steps: mixing polyethyleneand 5 to 45 weight % polypropylene to make a resin mixture; mixing 30 to75 weight % of an organic liquid material and 10 to 50 weight % of aninorganic material; and extracting the organic liquid material and theinorganic material. However, there still remains the problem associatedwith mixing the inorganic material as described above, even though theinorganic compound is extracted out. As already mentioned in the patentdocument, the addition of polypropylene which is not compatible withpolyethylene leads to deterioration of physical properties. Further,this procedure is complicated due to the additional steps of extractingand removing the inorganic compound. It requires a lot of polypropylenein order to improve thermal stability and, thus, further deterioratesthe physical property of the separator.

The method for coating a heat resistant material on the surface of amicroporous film is disclosed in U.S. Patent Application No.2006-0055075A1. Unfortunately, the coating system is restricted inincreasing the permeability of the coating layer. As a consequence, thefilm has poor permeability and the wetting property between the coatinglayer and the microporous film is likely to be poor to cause unevennessof quality.

In order to improve the thermal stability of the separator, laminationis used to prepare a multi-layered separator. In U.S. Pat. No.5,691,077, polyethylene outstanding in the shutdown property (i.e., lowmelting point) is laminated with polypropylene having a high melt downtemperature (i.e., high melting point) to fabricate a 3-layer-structuredseparator. This separator is outstanding in the thermal property, buthas a lot of disadvantages. In detail, this laminated separator isassociated with nonuniform stretching, pinhole generation, increaseddeviation of thickness, and the like, because films are prepared througha low-temperature dry process. It lowers productivity because of theadditional laminating step. Further, the separator may be delaminateddue to poor adhesion strength. As a consequence, this method is notapplied widely. In spite of superior thermal stability, this method doesnot satisfy the necessary requirements for the separator of a secondarybattery, such as strength, permeability, uniform quality andproductivity.

In Japanese Patent Laid-Open No. 2002-321323 and WO 2004/089627, themulti-layered separators that comprise a polyethylene microporous filmprepared under wet process as a main layer and layer comprised of amixture of polyethylene and polypropylene also prepared under amoisturized condition wet process as a surface layer, have beendisclosed. These separators are outstanding in the quality stability,because they are manufactured under wet process, but are restricted inthat they do not have a thermal stability better than that of thepolypropylene resin. Furthermore, the processes for preparing amulti-layered separator become complicated because all the layers of theseparator should be manufactured under wet process.

In WO 2006/038532, the multi-layered separator containing inorganicparticles prepared under wet process has been introduced. This separatorshould be fabricated under wet process, as described above. In additionto a complicated blending process, this method is associated with theproblem of deteriorated physical properties caused by the inclusion ofthe diluent, which has to be extracted out during film production, bymore than 50% on the surface layer (Stretching performed in the presenceof a diluent does not give desired effect).

In the separator of a secondary battery, the characteristics ofstrength, permeability and evenness of quality are essential, and,recently, superior thermal stability is also required. However, theconventional methods mentioned above cannot accomplish the requirementsof strength, permeability, thermal stability and evenness of qualitysimultaneously to the level of the separator prepared under wet process.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present invention has been made in an effort to solve theabove-described problems associated with the related art. The presentinventors attempted to stretch the mixture of polypropylene and aheat-resistant filler having a higher meting temperature than thepolypropylene without a diluent so that we prepared a porous film of thepolypropylene and the heat-resistant filler as the interface is crackedto generate pore. Then, the porous film was laminated with an additionalporous film prepared under wet process, in order to manufacture thenovel porous film that satisfies the physical property and the qualitystability of the polyethylene microporous film layer and the strengthalong with the permeability of the combined porous film layer of thepolypropylene and the heat-resistant filler. Therefore, we haveidentified that the separator of the present invention should beoutstanding in all of the strength, the permeability, evenness ofquality and the thermal stability, and have completed the inventionsuccessfully.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain example embodimentsthereof illustrated in the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is micropores fabricated by stretching and extracting afterseparating the phase of polyethylene and the diluent;

FIG. 2 is macropores fabricated by cracking the interface betweenpolypropylene and heat-resistant filler;

FIG. 3 is a frame having an outer size of 7.5 cm×7.5 cm and an innersize of 2.5 cm×2.5 cm to measure melt down temperature; and

FIG. 4 is a film having a size of 5 cm×5 cm installed at frame of FIG.3.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with example embodiments, it will be understoodthat the present description is not intended to limit the invention tothose example embodiments. On the contrary, the invention is intended tocover not only the example embodiments, but also various alternatives,modifications, equivalents and other embodiments, which may be includedwithin the spirit and scope of the invention as defined in the appendedclaims.

In one aspect, the present invention provides a microporous polyolefinmultilayer film that is prepared by the process comprising steps:

(a) melting and mixing mixture that comprises 20-50 weight % ofpolyethylene having a melting temperature of 125° C. or higher and 80-50weight % of a diluent;

(b) melting and mixing mixture that comprises 20-70 weight % ofpolypropylene having a melting temperature of 160° C. or higher and80-30 weight % of a heat-resistant filler having a melting temperatureof 170° C. or higher;

(c) forming the blended melt prepared in the (a) and (b) steps into amulti-layered sheet of 2 or 3 layers;

(d) forming a film by stretching the multi-layered sheet;

(e) extracting the diluent from the film; and

(f) heat setting the film,

wherein the pores of the polyethylene layer are micropores that areformed by stretching and extracting after separating the phase ofpolyethylene and the diluent and have an average size of 0.1 μm orsmaller; and the pores of the polypropylene and heat-resistant fillerlayer are macropores that are fabricated by cracking of the interfacebetween the polypropylene and heat-resistant filler and have an averagesize of 1-50 μm, and the microporous polyolefin multilayer film hasthickness of 9-30 μm, a piercing strength of 0.15 N/μm or more, apermeability of 1.5×10⁻⁵ Darcy or more and a melt fracture temperatureof 170° C. or higher.

The basic principle of the present invention for preparing themicroporous polyolefin multilayer film will be described as follows.

As described above, the method for preparing a microporous film by usingpolyethylene under wet process is comprised of steps: mixingpolyethylene and its matching diluent; extruding to manufacture a sheet;stretching to form a film; and extracting the diluent by using anorganic solvent to fabricate a porous film. However, because the meltingtemperature of the separator made of polyethylene cannot exceed 135° C.,it is limited in thermal stability. In contrast, the melting temperatureof polypropylene may be above 160° C. But, because of insufficientcrystallinity, highly-permeable films cannot be made under wet process.Furthermore, the shutdown temperature is high due to the high meltingtemperature, which results in deteriorated safety (As Shutdowntemperature is the temperature at which the micropores of the separatorare closed to shut the electric current when the inside of the batteryis abnormally overheated, the lower is the better). When being blendedwith polypropylene or a heat-resistant filler in order to overcome thisproblem, polyethylene can be improved for the thermal stability (i.e.,increased melt fracture temperature) as described above. But, theheat-resistant material cannot increase the melt fracture temperaturesufficiently because it exists in a particulate form and is notinterconnected with the polyethylene matrix. Moreover, this materialdeteriorates the whole physical property due to weak interface withpolyethylene.

Therefore, a polyethylene layer and a polypropylene or heat-resistantfiller layer should be manufactured separately and then combined to forma multi-layered separator, rather than blending polyethylene andpolypropylene or the heat-resistant filler. In this case, thepolypropylene is continuously connected in the polypropylene andheat-resistant filler layer and the heat-resistant filler exists in theheat-resistant filler and polypropylene matrix in a particulate form. Asa consequence, the porous film that has the low shutdown temperature ofpolyethylene, the high melting temperature of polypropylene and the goodthermal stability of inorganic material along with high melt fracturetemperature can be manufactured.

When the polypropylene and heat-resistant filler are simply mixed, theydo not have permeability. In order to confer the permeability to thepolypropylene and heat-resistant filler layer, the stretching process isapplied during stretching of polyethylene under a moisturized condition,in the present invention. In detail, a sheet made of the mixture ofpolyethylene and a diluent and a sheet made of the mixture ofpolypropylene and a heat-resistant filler are combined to form amulti-layered sheet and stretched to crack the interface betweenpolypropylene and heat-resistant filler in their mixture layer and toform pores. Thereafter, the diluent of the polyethylene layer isextracted out by the extracting process. As a consequence, themulti-layered microporous polyolefin film that has all theproperties—the stability of polyethylene separator prepared under wetprocess, and the thermal stability, electrolytic impregnability,strength and permeability of the polypropylene and heat-resistantfiller—is manufactured.

The pores of the separator prepared above have followingcharacteristics. The pores of the polyethylene layer are microporesformed by stretching and extracting after separating the phase ofpolyethylene and the diluent (see FIG. 1) and have an average size of0.1 μm or smaller. The pores of the polypropylene and heat-resistantfiller layer are macropores fabricated by cracking of the interfacebetween polypropylene and the inorganic material (see FIG. 2) and havean average size of 1-50 μm.

In another aspect, the present invention provides a method for preparinga microporous polyolefin multilayer film which comprises steps:

(a) melting and mixing mixture that comprises 20-50 weight % ofpolyethylene having a melting temperature of 125° C. or higher and 80-50weight % of a diluent;

(b) melting and mixing mixture that comprises 20-70 weight % ofpolyethylene having a melting temperature of 160° C. or higher and 80-30weight % of a heat-resistant filler having a melting temperature of 170°C. or higher;

(c) forming the blended melt prepared in the (a) and (b) steps into amulti-layered sheet of 2 or 3 layers;

(d) forming a film by stretching the multi-layered sheet;

(e) extracting the diluent from the film; and

(f) heat setting the film.

In the step (a) melting and mixing mixture that comprises 20-50 weight %of polyethylene having a melting temperature of 125° C. or higher and80-50 weight % of a diluent, the polyethylene used in the invention issingle or combined polyethylene comprising ethylene only or ethylene andC₃-C₈ α-olefin and having a melting temperature of 125° C. or higher.The combined polyethylene is a mixture of ethylene only or ethylene andC₃-C₈ α-olefin co-monomer, and has a melting temperature of 125° C. orhigher. The C₃-C₈ α-olefin co-monomer may be propylene, 1-butene,1-hexene, 4-methylpetene-1, and the like. The weight average molecularweight of the polyethylene is preferably 200,000-3,000,000. When theweight average molecular weight is under 200,000, the final physicalproperty of the porous film becomes worse. When the weight averagemolecular weight is over 3,000,000, the productivity becomes lowerbecause extruding and mixing property is worse. The weight averagemolecular weight is more preferably 200,000-1,500,000.

The diluent may be all kinds of organic liquids capable of forming asingle phase with the resin at the temperature of extrusion. It may bean aliphatic or cyclic hydrocarbon such as nonane, decane, decalin,paraffin oil, etc. or a phthalic acid ester such as dibutyl phthalateand dioctyl phthalate, etc. The diluent may be paraffin oil that isnontoxic to human body, has a high boiling point and is low in volatilecomponents, and preferably, the diluent may be paraffin oil having akinetic viscosity of 20-200 cSt at 40° C. When the kinetic viscosity isover 200 cSt, the paraffin oil becomes problematic to increase load anddeteriorate the surface of the sheet or film. Further, it may complicatethe extraction process, decrease the productivity and reduce thepermeability due to remained oil. When the kinetic viscosity is under 20cSt, the paraffin oil is difficult to be mixed during the extrusionbecause it has a viscosity quite different from that of the meltedpolyethylene inside the extruder.

The composition of polyethylene and the diluent may be comprised of20-50 weight % of polyethylene and 80-50 weight % of the diluent. Whenthe content of polyethylene goes beyond 50 weight % (i.e., when thecontent of the diluent is under 50 weight %), the porosity, the poresize and the interconnection between pores decrease to drop down thepermeability significantly. In contrast, when the content ofpolyethylene is under 20 weight % (i.e., when the content of diluent isover 80 weight %), the blending property of polyethylene and the diluentdecreases so that they are not mixed thermodynamically to be extruded ina gel form. This sheet may result in rupture during stretching andnonuniform thickness.

If necessary, typical additives such as oxidation stabilizers, UVstabilizers, antistatics, etc. may be added in order to improve specificfunctions.

The mixture of polyethylene and the diluent may be melted and mixed byusing a twin screw compounder, blender, Banbury mixer, etc. speciallydesigned for the blending of the diluent with the polyethylene. Themelting and mixing may be conducted at 180-300° C. The polyethylene andthe diluent may be previously mixed and poured into an extruder, orpoured separately from different feeders.

In the step (b) melting and mixing mixture that comprises 20-70 weight %of polypropylene having a melting temperature of 160° C. or higher and80-30 weight % of a heat-resistant filler having a melting temperatureof 170° C. or higher, the polypropylene is single or combinedpolypropylene comprising propylene only or propylene, ethylene and C₄-C₈α-olefin having a melting temperature of 160° C. or higher. The combinedpolypropylene is a mixture of propylene only or propylene, ethylene andC₄-C₈ α-olefin and has a melting temperature of 160° C. or higher. Themolecular weight of polypropylene may be 50,000-2,000,000. When theweight average molecular weight is under 50,000, the physical propertyof the porous film becomes bad although mixing performance with theinorganic material is good. When the weight average molecular weight isover 2,000,000, the extruding and mixing property becomes poor.

The heat-resistant filler plays the following three roles: First, theheat-resistant filler plays the role as nucleus to form pores at theinterface during the stretching after being mixed with polypropylene.Second, the heat-resistant filler plays the role of increasing thethermal stability of the porous film. Third, the heat-resistant fillerplays the role of improving electrolytic absorption because it has anoutstanding affinity with the electrolyte solution due to its inherentpolarity. The heat-resistant filler may be a polar heat-resistant resinselected among polyvinylidene fluoride, polymethylpentene, polyethyleneterephthalate, polycarbonate, polyester, polyvinyl alcohol,polyacrylonitrile and polymethylene oxide. The heat-resistant filler maybe an inorganic material having an average particle size of 0.01-5 μmand may be selected among silicon dioxide (SiO₂), aluminum oxide(Al₂O₃), calcium carbonate (CaCO₃), titanium dioxide (TiO₂), SiS₂,SiPO₄, MgO, ZnO, BaTiO₃, natural or organically modified clay and amixture thereof.

With respect to the heat-resistance, the initial size of the polarheat-resistant resin is not an important factor because it is melted andmixed with polypropylene, but the particle size of the inorganicmaterial is very important. The average particle size of the inorganicmaterial may be 0.01-5 μm. When the average particle size is smallerthan 0.01 μm, the pore size becomes smaller after stretching and is notsuitable for the porous film. When the average particle size is largerthan 5 μm, the pore size becomes larger after stretching so as todeteriorate the physical property of the porous film. Further, it mayaffect the whole physical property of the separator according to thepresent invention, which has a thickness of 9-30 μm.

The mixture of polypropylene and the heat-resistant filler may becomprised of 20-70 weight % of polypropylene and 80-30 weight % of theheat-resistant filler. When the content of polypropylene goes over 70weight %, the number of pores generated during stretching decreases todrop down the permeability significantly. In contrast, when the contentof polypropylene is under 20 weight %, the polypropylene cannot beinterconnected in a matrix form, thereby decreasing the physicalproperty of the porous film and failing to improve heat resistance. Thevolume ratio of polypropylene and the heat-resistant filler may be mosteffective at 50/50. When considering the higher density of theheat-resistant filler, the composition of polypropylene andheat-resistant filler may be 30 to 50 weight % of polypropylene and 70to 50 weight % of the heat-resistant filler.

If necessary, typical additives such as oxidation stabilizers, UVstabilizers, electrostatics, etc. may be added in order to improvespecific functions.

The mixture of polypropylene and heat-resistant filler may be melted andmixed by using a twin screw compounder, blender, Banbury mixer, etc.specially designed for the blending of the heat-resistant filler and thepolyolefin. When the heat-resistant filler is a polar heat-resistantresin, the blending process may be conducted at 30-50° C. highertemperature than the melting temperature of the polar heat-resistantresin. When the blending temperature is lower than the temperaturerange, the polar heat-resistant resin may not be melted and blendedsufficiently. In contrast, when the blending temperature is higher thanthe temperature range, the resin including polypropylene may beexcessively thermally oxidized. The blending process may be conducted at180-250° C., when the heat-resistant filler is an inorganic material.The polypropylene and the polar heat-resistant filler may be previouslymixed and poured into a compounder, or poured separately from differentfeeders.

In the step (c) forming the blended melt prepared in the (a) and (b)steps into a multi-layered sheet of 2 or 3 layers, typical casting orcalendaring methods may be used to form the melt into a sheet. Thetemperature of casting or calendaring roll may be 30-80° C. When thetemperature of casting a roll is under 30° C., the sheet may generatewrinkles due to rapid cooling. In contrast, when the temperature ofcasting roll is above 80° C., the sheet may have poor surface due toinsufficient cooling.

In order to form the multi-layered sheet, general co-extrusion method,heat adhesion method or coating method may be adopted. The co-extrusionmethod is a method for preparing a multi-layered sheet, in which twokinds of melt extruded from each extruder are co-extruded through amulti-layered T die to fabricate the multi-layered sheet. The heatadhesion method is a method for preparing a multi-layered sheet, inwhich sheets extruded from different extruders are heat adhered underpressure. The coating method is a method for preparing a multi-layeredsheet, in which a first sheet is extruded over another sheet layer.

In the step (d) forming a film by stretching the multi-layered sheet, inorder to stretch sheet, any kinds of stretching method may be used. Forexample, a tenter-type simultaneous stretching may be used, or aconsecutive stretching of stretching first in the longitudinal directionusing a roll and then stretching in the transverse direction using atenter may be used. The stretch ratio may be more than 4 times in thelongitudinal and transverse directions, respectively, and the totalstretch ratio may be 25-60 times. When the stretch ratio of in onedirection is less than 4 times, the physical property balance in thelongitudinal and transverse directions is broken due to the insufficientstretching in one direction, and the puncture strength may decreases.When the total stretch ratio is less than 25 times, the stretching maybe insufficient. In contrast, when the total stretch ratio exceeds 60times, the film may be ruptured during the stretching the shrinkageratio of the film may increase.

The stretching temperature may vary depending on the melting point ofthe polyethylene and the concentration and kinds of the diluent. Theoptimal temperature may be selected such that 30-80 weight % of thecrystalline portion of the polyethylene and diluent layer in themultilayered sheet is melted. When the stretching temperature is lowerthan the temperature at which 30 weight % of the crystalline portion ofthe polyethylene and diluent layer within the multi-layered sheet ismelted, the film may lack softness. As a consequence, the film may beruptured during stretching or be stretched insufficiently. In contrast,when the stretching temperature is higher than the temperature range atwhich 80 weight % of the crystalline portion of the polyethylene anddiluent layer within the multi-layered sheet is melted, the film isstretched easily but the variation of thickness increases due to partialover-stretching. Further, the physical property of the resin may bedeteriorated due to insufficient orientation effect. The above-saidstretching temperature is lower than the melting temperature of thepolypropylene, but the polypropylene can be stretched at the temperaturerange. Through the stretching, the polypropylene is stretched withoutbeing ruptured to crack the interface between polyethylene and theheat-resistant filler. Simultaneously, pores are produced in thepolypropylene and the heat-resistant filler layer.

Because the stretching can be attained without using a diluent, thestretching effect and the overall physical properties of the separatorare improved.

The degree of melting of the crystalline portion can be analyzed bydifferential scanning calorimetry (DSC).

In the step (e) extracting the diluent from the film, the thin sheetprepared after the stretching, i.e., the film, is dried after extractingthe diluent using an organic solvent. Any kind of organic solvent may beused if it can extract out the diluent used to extrude the resin.Suitable organic solvents include methyl ethyl ketone, methylenechloride, hexane, etc., which provide good extraction efficiency and aredried quickly. The extraction can be performed by any common solventextraction method, including immersion, solvent spraying,ultrasonication, or the like, alone or in combination. The residualdiluent content after the extraction should be not more than 1 weight %.If the residual diluent content exceeds 1 weight %, physical propertiesmay be deteriorated and film permeability may decrease.

The amount of the residual diluent varies a lot depending on theextraction temperature and extraction time. A high extractiontemperature is preferred when considering solubility of the diluent inthe solvent. But, an extraction temperature of 40° C. or below ispreferred when considering the safety issue associated with the boilingof the solvent. The extraction temperature should be higher than thesolidification point of diluent because the extraction efficiencydecreases remarkably if it is below the solidification point of diluent.The extraction time may vary depending on the film thickness. In case ofa usual microporous film having a thickness of 9-30 μm, an extractiontime of 2-4 minutes will be adequate.

In the step (f) heat setting the film, the film dried above is heat-set.That is, residual stress is removed so as to reduce the shrinkage ratioof the film. In the heat setting process, the film tending towardshrinkage is forcibly fixed, stretched or shrunk while heating so as toeliminate the residual stress. A higher heat-setting temperature ispreferred with respect to lowering the shrinkage ratio. However, whenthe heat-setting temperature is excessively high, permeability maydecrease because the micropores can be closed as the film is partiallymelted. Preferably, the heat-setting temperature is selected such that10-70 weight % of the polyethylene crystalline portion of the film ismelted. If the heat-setting temperature is lower than the temperature atwhich 10 weight % of the crystalline portion of the film is melted, theeffect of the elimination of residual stress may be insignificantbecause the reorientation of the polyethylene molecules in the film isinsufficient. And, if the heat-setting temperature is higher than thetemperature at which 70 weight % of the crystalline portion of the filmis melted, permeability may decrease because the micropores may beclosed due to the partial melting.

The heat-setting time may be relatively short if the heat-settingtemperature is high, and may be relatively long if the heat-settingtemperature is low. When a tenter-type continuous heat-setting apparatusis used, a heat-setting time of 20 seconds to 2 minutes will beadequate. Most preferably, the heat-setting time may be 1-2 minutes inthe temperature range where 10-30 weight % of the crystalline portion ofthe film is melted, and 20 seconds to 1 minute in the temperature rangewhere 30-70 weight % of the crystalline portion of the film is melted.

The microporous polyolefin multilayer film manufactured by theabove-described process will be described more clearly as follows.

The microporous polyolefin multilayer film is comprised of apolyethylene layer having a melting temperature of 125° C. or higher anda combined polypropylene and inorganic material layer comprising 20-70weight % of polypropylene having a melting temperature of 160° C. orhigher and 80-30 weight % of an heat resistant filler having a meltingtemperature of 160° C. or higher. It is a 2- or 3-layered microporousfilm. In case of a 3-layered film, the polyethylene layer may be amiddle layer and the polypropylene and heat-resistant filler layer isboth surface layers, and vice versa.

Preferably, the polyethylene layer may occupy more than 50% of the totalthickness and the polypropylene and heat-resistant filler layer has athickness of at least 1 μm. When the polyethylene layer occupies lessthan 50% of the total thickness, the separator, prepared under wetprocess, which has evenness of quality and physical property due touniform pore size occupies less than 50% so that overall evenness ofseparator quality may be deteriorated. And, preferably, thepolypropylene and heat-resistant filler layer which contributes to theimprovement of thermal stability has a thickness of at least 1 μm. Whenthe thickness is smaller than 1 μm, improvement of thermal stability maybe insufficient.

The multi-layered microporous film of the present invention has athickness of 9-30 μm, puncture strength of 0.15 N/μm or more and apermeability of 1.5×10⁻⁵ Darcy or more, melt fracture temperature of170° C. or more. When the thickness of the film is under 9 μm, theoverall strength of the separator becomes weak so that this is notproper for the separator of a secondary battery. When the thickness ofthe film is over 30 μm, the permeability of the separator becomes low sothat this is not proper for the separator of a secondary battery.

The puncture strength of the film may be more than 0.15 N/μm. When thepuncture strength is below 0.15 N/μm, the piercing strength is so weakthat the film is improper for the separator of a secondary battery. Morepreferably, the puncture strength of the film may be 0.2-0.5 N/μm.

The gas permeability of the film may be more 1.5×10⁻⁵ Darcy or more. Agas permeability below 1.5×10⁻⁵ Darcy is insufficient to manufacture ahigh-capacity, high power battery. More preferably, the gas permeabilityof the film may be 2.5-10.0×10⁻⁵ Darcy.

The melt fracture temperature of the film varies depending upon themelting temperature of polypropylene and the content of the inorganicmaterial and may be 170-220° C. Thermal stability test of a battery isusually performed at 150° C., but the temperature of a battery mayincrease higher when an internal short circuit occurs. Therefore, it ispreferable that the melt fracture temperature of the film is 170° C. orhigher.

The pores of the porous film prepared in the present invention exist intwo forms. The pores of the polyethylene layer are micropores that aremanufactured by stretching and extracting after separating the phase ofpolyethylene and the diluent and have an average size of 0.1 μm orsmaller. When the average size is over 0.1 μm, the pores badly affectthe overall safety and the stability of the film. Preferably, themicropores may have an average size of 0.01-0.1 μm. In contrast, thepores of the polypropylene and heat-resistant filler layer aremacropores that are fabricated by cracking of the interface between thepolypropylene and the heat resistant filler and have an average size of1-50 μm. When the average size is smaller than 1 μm, the pores mayreduce the permeability of the whole film. When the average size islarger than 50 μm, the pores may reduce the physical property of thefilm and drop down the thermal stability. The thermal stability can bemaximized when the pores having an average size of 50 μm or smaller areuniformly distributed to form a stable network of polypropylene. Thepores of the polypropylene and heat-resistant filler layer may have anaverage size of 1-20 μm more preferably.

The microporous multi-layered film of the present invention has afeature to have both the low shutdown temperature caused by thepolyethylene and the high melt fracture temperature caused by thepolypropylene and the heat-resistant filler. In addition, it has thehigh strength and stability conferred by the micropores prepared underwet process and the high permeability and high strength conferred by themacropores prepared under dry process. Therefore, this multi-layeredfilm can be used effectively to manufacture a secondary battery withhigh capacity and high power.

EXAMPLES

The following examples illustrate the invention and are not intended tolimit the same. The molecular weight and the distribution of molecularweights are measured by using a high-temperature GPC (gel permeationchromatography) system (Polymer Laboratory). The kinetic viscosity ofthe diluent is measured with CAV-4 Automatic Viscometer (Cannon). Thesheets and the films are manufactured from raw materials by thefollowing procedures.

Preparation of Films

Polyethylene and a diluent were melted and mixed with a twin screwcompounder having Φ=46 mm. The mixing temperature was 180-240° C. Thepolyethylene was added with a main feeder and the diluent was pouredinto an extruder by using a side-feeder. The blended melt was extrudedon the T- die and molded with a casting roll at 30° C. to have properthickness.

Polypropylene and a heat-resistant filler were mixed and extruded with atwin screw compounder having Φ=30 mm. The mixing temperature was220-330° C. The polypropylene and the heat-resistant filler were blendedpreviously and poured into an extruder. The blended and extruded meltwas extruded with another extruder on the T- die and molded with acasting roll at 30° C. to have proper thickness.

In order to prepare a multi-layered sheet, each sheet was laminated witha compression molder with heat. The adhesion temperature was 200° C. andthe heat fusion period was 30 seconds.

In order to analyze the melting phenomenon of the crystalline portion ofthe molded sheet according to temperatures, DSC (Mettler Toledo) wasused. The analytic condition was: sample weight=5 mg; and scanningrate=10° C./min.

Sheets were simultaneously stretched with a tenter-type lab stretcher,while varying the stretching ratio and stretching temperature. Thestretching temperature was determined in the temperature range where30-80 weight % of crystalline portion of the polyethylene and diluentlayer were melted, as determined by DSC.

The diluent was extracted out for 5 minutes using methylene chloride bythe immersion method.

Heat setting was performed in a convection oven, after the extractedfilm was dried and fixed on a frame. The heat setting was conducted at120° C. for 1 minute and 30 seconds.

The thickness of each film layer was measured with a scanning electronmicroscope (SEM). The prepared film was cooled under a liquid nitrogencondition for 20 seconds, then ruptured instantly and observed on thecross-section so as to measure the thickness.

The average size of the pores in each layer was measured by thefollowing two procedures. In order to measure the pore size of thepolyethylene and diluent layer, a singlelayer film was manufacturedunder the same condition and measured with a porometer (PMI) by usingthe half-dry method according to ASTM F316-03. The pore size of thepolypropylene and heat-resistant filler layer was measured by observingthe film surface with an electron microscope. The film manufacturedabove was examined to measure the most important properties for amicroporous film, that is, the puncture strength, gas permeability andmelt fracture temperature. The result is summarized in Table 1.

Measurement of Physical Properties

(1) The puncture strength was a strength of the film when it was piercedby a pin having a diameter of 1.0 mm at a speed of 120 mm/min.

(2) The gas permeability was measured with a porometer (PMI,CFP-1500-AEL). The gas permeability is usually denoted with Gurleynumber, but the Gurley number is inconvenient in finding out therelative permeability associated with the pore structure of theseparator itself, because it does not consider the effect by thethickness of the separator. To avoid this problem, Darcy's permeabilityconstant was used, instead. Darcy's permeability constant is calculatedby the following Equation 1. Nitrogen was used for the measurement.C=(8 FTV)/[πD²(P²−1)]  Equation 1

where

C=Darcy's permeability constant,

F=flow rate,

T=sample thickness,

V=viscosity of gas (0.185 for N₂),

D=sample diameter, and

P=pressure.

In the present description, an average of Darcy's permeability constantin the region 100-200 psi was used.

(3) In order to measure the melt fracture temperature of the film, thefilm (5 cm×5 cm, FIG. 4) was fixed on a frame (outer size=7.5 cm×7.5 cm;inner size=2.5 cm×2.5 cm, FIG. 3) using a polyimide tape, and allowed tostand for 10 minutes in a convection oven maintained at a presettemperature. It was observed whether the film was ruptured or not. Themelt fracture temperature was defined as the highest temperature atwhich the film was not ruptured even after 10 minutes.

Example 1

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C. and paraffin oil having akinetic viscosity of 95 cSt at 40° C. were used, and the proportion ofthe two components was 30 weight % and 70 weight %, respectively. Thethickness of the layer 1 sheet prepared above was 950 μm. In layer 2,polypropylene having a weight average molecular weight of 5.7×10⁵ and amelting temperature of 163° C. and CaCO₃ having an average particle sizeof 1.5 μm were used, and the proportion of the two components was 50weight % and 50 weight %, respectively. The thickness of the layer 2sheet prepared above was 100 μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 119°C. to a total stretching ratio of 36 times, 6 times in the longitudinaldirection and 6 times in the transverse direction. The film thicknesswas 22 μm after the extraction and heat setting of the film.

Example 2

In layer 1, polyethylene having a weight average molecular weight of2.7×10⁵ and a melting temperature of 130° C. because of using propyleneas co-monomer and paraffin oil having a kinetic viscosity of 95 cSt at40° C. were used, and the proportion of the two components was 30 weight% and 70 weight %, respectively. The thickness of the layer 1 sheetprepared above was 1,000 μm. In layer 2, polypropylene having a weightaverage molecular weight of 2.5×10⁵ and a melting temperature of 1 60°C. because of using ethylene as co-monomer and SiO₂ having an averageparticle size of 3.0 μm were used, and the proportion of the twocomponents was 30 weight % and 70 weight %, respectively. The thicknessof the layer 2 sheet prepared above was 90 μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 119°C. to a total stretching ratio of 49 times, 7 times in the longitudinaldirection and 7 times in the transverse direction. The film thicknesswas 17 μm after the extraction and heat setting of the film.

Example 3

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C. and paraffin oil having akinetic viscosity of 95 cSt at 40° C. were used, and the proportion ofthe two components was 30 weight % and 70 weight %, respectively. Thethickness of the layer 1 sheet prepared above was 700 μm. In layer 2,polypropylene having a weight average molecular weight of 2.5×10⁵ and amelting temperature of 163° C. and polymethylpentene were used, and theproportion of the two components was 70 weight % and 30 weight %,respectively. The thickness of the layer 2 sheet prepared above was 220μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 122°C. to total stretching ratio of 36 times, 6 times in the longitudinaldirection and 6 times in the transverse direction. The film thicknesswas 20 μm after the extraction and heat setting of the film.

Example 4

In layer 1, polyethylene having a weight average molecular weight of2.7×10⁵ and a melting temperature of 130° C. because of using propyleneas co-monomer and paraffin oil having a kinetic viscosity of 95 cSt at40° C. were used, and the proportion of the two components was 50 weight% and 50 weight %, respectively. The thickness of the layer 1 sheetprepared above was 340 μm. In layer 2, polypropylene having a weightaverage molecular weight of 5.7×10⁵ and a melting temperature of 163° C.and polycarbonate were used, and the proportion of the two componentswas 60 weight % and 40 weight %, respectively. The thickness of thelayer 2 sheet prepared above was 35 μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 122°C. to a total stretch ratio of 25 times, 5 times in the longitudinaldirection and 5 times in the transverse direction. The film thicknesswas 12 μm after the extraction and heat setting of the film.

Example 5

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C. and paraffin oil having akinetic viscosity of 95 cSt at 40° C. were used, and the proportion ofthe two components was 20 weight % and 80 weight %, respectively. Thethickness of the layer 1 sheet prepared above was 2,000 μm. In layer 2,polypropylene having a weight average molecular weight of 2.5×10⁵ and amelting temperature of 163° C. and BaTiO₃ having an average particlesize of 0.4 μm were used, and the proportion of the two components was20 weight % and 80 weight %, respectively. The thickness of the layer 2sheet prepared above was 80 μm.

The layers 1 and 2 were heat fused to make a 3-layered structure in theorder of layer 2, layer 1 and layer 2, and stretched simultaneously at122° C. to a total stretching ratio of 49 times, 7 times in longitudinaldirection and 7 times in the transverse direction. The film thicknesswas 30 μm after the extraction and heat setting of the film.

Comparative Example 1

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C. and paraffin oil having akinetic viscosity of 95 cSt at 40° C. were used, and the proportion ofthe two components was 30 weight % and 70 weight %, respectively. Thethickness of the layer 1 sheet prepared above was 1,100 μm.

The layer 1 was stretched simultaneously at 120° C. to a totalstretching ratio of 36 times, 6 times in the longitudinal direction and6 times in the transverse direction. The film thickness was 20 μm afterthe extraction and heat setting of the film.

Comparative Example 2

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C. and paraffin oil having akinetic viscosity of 95 cSt at 40° C. were used, and the proportion ofthe two components was 60 weight % and 40 weight %, respectively. Thethickness of the layer 1 sheet prepared above was 500 μm. In layer 2,polypropylene having a weight average molecular weight of 2.5×10⁵ and amelting temperature of 163° C. was used alone. The thickness of thelayer 2 sheet prepared above was 200 μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 116°C. to a total stretch ratio of 36 times, 6 times in the longitudinaldirection and 6 times in the transverse direction. The film thicknesswas 18 μm after the extraction and heat setting of the film.

Comparative Example 3

In layer 1, polyethylene having a weight average molecular weight of3.0×10⁵ and a melting temperature of 134° C., CaCO₃ having an averageparticle size of 1.5 μm and paraffin oil having a kinetic viscosity of95 cSt at 40° C. were used. The mixture was mixed and extruded with atwin screw compounder having Φ=46 mm at 180-240° C. The polyethylene andthe CaCO₃were previously mixed and poured into an extruder. The paraffinoil was injected into the extruder by using a side-feeder. Theproportions of the three components were: 21 weight % polyethylene, 9weight % CaCO₃ and 70 weight % paraffin oil. The blended melt wasextruded on the T-type die and molded with a casting roll into a sheethaving a thickness of 450 μm.

In layer 2, polypropylene having a weight average molecular weight of5.7×10⁵ and a melting temperature of 163° C. and polyethylene having aweight average molecular weight of 3.0×10⁵ and a melting temperature of134° C. were used. The mixture was blended and extruded with a twinscrew compounder having Φ=46 mm at 180-240° C. The polypropylene and thepolyethylene were previously mixed and poured into the extruder. Theproportion of the two components was 80 weight % and 20 weight %,respectively. The thickness of the layer 2 sheet prepared above was 480μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 120°C. to a total stretching ratio of 36 times, 6 times in the longitudinaldirection and 6 times in the transverse direction. The film thicknesswas 22 μm after the extraction and heat setting of the film.

Comparative Example 4

In layer 1, polyethylene having a weight average molecular weight of2.7×10⁵ and a melting temperature of 130° C. because of using propyleneas co-monomer and paraffin oil having a kinetic viscosity of 95 cSt at40° C. were used, and the proportion of the two components was 30 weight% and 70 weight %, respectively. The thickness of the layer 1 sheetprepared above was 180 μm. In layer 2, polypropylene having a weightaverage molecular weight of 2.0×10⁵ and a melting temperature of 132° C.because of using ethylene and 1-butene as co-polymer and SiO₂ having anaverage particle size of 5.5 μm were used. The proportion of the twocomponents was 50 weight % and 50 weight %, respectively. The thicknessof the layer 2 sheet prepared above was 230 μm.

The layers 1 and 2 were heat fused and stretched simultaneously at 118°C. to a total stretching ratio of 24.5 times, 7 times in thelongitudinal direction and 3.5 times in the transverse direction. Thefilm thickness was 15 μm after the extraction and heat setting of thefilm.

Comparative Example 5

In layer 1, polyethylene having a weight average molecular weight of1.5×10⁵ and a melting temperature of 121° C. because of using 1-buteneas co-monomer and paraffin oil having a kinetic viscosity of 95 cSt at40° C. were used, and the proportion of the two components was 30 weight% and 70 weight %, respectively. The thickness of the layer 1 sheetprepared above was 750 μm. In layer 2, polypropylene having a weightaverage molecular weight of 2.5×10⁵ and a melting temperature of 163° C.and polycarbonate were used, and the proportion of the two componentswas 10 weight % and 90 weight %, respectively. The thickness of thelayer 2 sheet prepared above was 25 μm.

The layers 1 and 2 were heat fused and heating, then tried to bestretched at 110-120° C. But, stretching was impossible because ruptureoccurred.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Layer 1 Resin PE(134) PE(130)PE(134) PE(130) PE(134) (Tm, ° C.) Layer 2 Resin PP(163) PP(160) PP(163)PP(163) PP(163) (Tm, ° C.) Filler (size, CaCO₃(1.5) SiO₂(3.0) PMP PCBaTiO₃(0.4) μm) Resin/filler 50/50 30/70 70/30 60/40 20/80 (wt %/wt %)Structure Layer1/2 Layer1/2 Layer1/2 Layer1/2 Layer2/1/2 Total thickness(μm) 22 17 20 12 30 PE Thickness 81 82 58 85 89 layer ratio (%) Poresize 0.024 0.028 0.032 0.015 0.038 (μm) PP/filler Thickness 4.2 3.1 8.41.8 3.3 layer (μm) Pore size 25 38 22 15 4 (μm) Puncture strength 0.250.22 0.15 0.19 0.21 (N/μm) Permeability (10⁻⁵ 1.9 2.7 3.4 1.6 3.5 Darcy)Melt fracture 177 173 175 170 179 temperature (° C.) PE: polyethylene;PP: polypropylene; PMP: polymethylpentene; PC: polycarbonate; Tm:melting temperature.

TABLE 2 Comp. Ex. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Comp.Ex. 5 Layer 1 Resin (Tm, ° C.) PE(134) PE(134) PE(134) PE(130) PE(121)Filler (size, μm) — — CaCO₃ — — (1.5) Resin/filler — — 70/30 — — (wt%/wt %) Layer 2 Resin (Tm, ° C.) — PP(163) PP(163) PP(132) PP(163)PE(134) Filler (size, μm) — — — SiO₂ PC (5.5) Resin/filler — — PP/PE =80/20 50/50 10/90 (wt %/wt %) Structure Layer1 Layer1/2 Layer1/2Layer1/2 Layer1/2 Total thickness (μm) 20 18 22 15 Stretching impossiblePE layer Thickness ratio 100 71 42 31 — (%) Pore size (μm) 0.025 0.012.8 0.042 — PP/filler Thickness (μm) — 5.3 12.8 10.3 — layer Pore size(μm) — No pore 18 12 — Puncture strength (N/μm) 0.24 0.32 0.07 0.05 —Permeability (10⁻⁵ Darcy) 2.0 — 1.2 3.2 — Melt fracture temperature 135165 165 137 — (° C.) PE: polyethylene; PP: polypropylene; Tm: meltingtemperature.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the accompanying claims and their equivalents.

1. A microporous polyolefin multilayer film comprising a polyethylenelayer comprising a polyethylene having a melting temperature of 125° C.or higher, and a polypropylene and heat-resistant layer comprises 20-60wt % of propylene having a melting temperature of 160° C. or higher and80-40 wt % of a heat-resistant resin having a melting temperature of170° C. or higher, wherein the film has a thickness of 9-30 μm, apiercing strength of 0.15 N/μm or more, a permeability of 1.5×10⁻⁵ Darcyor more and a melt fracture temperature of 170° C. or higher, in whichthe pores of the polyethylene layer are micropores that are fabricatedby stretching and extracting after separating the phase of polyethyleneand the diluent and have an average size of 0.1 μm or smaller; and thepores of the polypropylene and heat-resistant layer are macropores thatare fabricated by cracking of the interface between the polypropyleneand heat-resistant resin and have an average size of 1-50 μm.
 2. Themicroporous polyolefin multilayer film according to claim 1, where inthe polyethylene is single or combined polyethylene comprising ethyleneonly or ethylene and C3-C8 α-olefin and having a melting temperature of125° C. or higher; the polypropylene is single or combined polypropylenecomprising propylene only or propylene, ethylene and C4-C8 α-olefinhaving a melting temperature of 160° C. or higher; and theheat-resistant resin having a melting temperature of 170° C. or higheris a resin selected among polyvinylidene fluoride, polymethylpentene,polyethylene terephthalate, polycarbonate, polyester, polyvinyl alcohol,polyacrylonitrile and polymethylene oxide.
 3. The microporous polyolefinmultilayer film according to claim 1, wherein the polyethylene layeroccupies more than 50% of the total thickness and the polypropylene andheat-resistant layer has a thickness of at least 1 μm.
 4. Themicroporous polyolefin multilayer film according to claim 1, wherein thefilm has a thickness of 9-30 μm, a piercing strength of 0.2 N/μm or moreand a permeability of 2.5×10⁻⁵−10.0×10⁻⁵ Darcy.
 5. The microporouspolyolefin multilayer film according to claim 1, wherein thepolypropylene and heat resistant layer further comprises an inorganicmaterial that has an average particle size of 0.01-5 μm and is selectedamong silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), calcium carbonate(CaCO₃), titanium dioxide (TiO₂), SiS₂, SiPO₄, MgO, ZnO, BaTiO₃, naturalor organically modified clay, and a mixture thereof.